The present invention relates to a distributed constant element using a magnetic thin film and, more particularly, to a magnetic thin-film device used as an inductor for switching power supplies, a noise filter, a reception circuit for receiving a quasi-microwave such as a portable telephone and a PHS (Personal Handy-phone System), and a magnetic sensor.
The market for portable electronic apparatuses represented by a notebook personal computer, a portable telephone, a PHS, an electronic notebook has recently been expanded worldwide. These electronic apparatuses are decreased in size and increased in performance due to the progress of technology for semiconductor integrated circuits. As is well-known, a semiconductor integrated circuit includes integrated devices such as transistors, capacitors and resistors. Such an integrated circuit is improving in packing density year by year and rapid advances are being made in one-chip techniques. However, a magnetic element such as an inductance and a transformer is difficult to miniaturize and make thin, though it is important for fulfilling a function such as energy storage, impedance matching and filtering. The magnetic element is therefore a great hindrance to improvement in the degree of integration of integrated circuits.
In contrast, as one method for making a magnetic element smaller and thinner, there is proposed a magnetic thin-film device which can be manufactured by a process similar to that of forming a semiconductor integrated circuit. The development of magnetic thin-film devices is described in detail in Kimisuke SHIRAE et al., Micro-magnetic Devices, 1st ed., Institute of Industrial Research of Japan, 1992 and in Recent Advance in Micro-Magnetic Engineering, National Convention of Institute of Electric Engineering of Japan, 1997. In the development of a magnetic thin-film device, it was first investigated to apply the device to a switching power supply having an operating frequency of one MHz to several tens of MHz. Recently, an inductance element (quasi-microwave band inductor) for mobile communication, whose operating frequency ranges from 1 GHz to 2 GHz, has been developed actively. The application of magnetic thin-film devices to a switching power supply and the development of a quasi-microwave band inductor were announced in a magnetic meeting of the Institute of Electric Engineering of Japan which was held in February of 1997.
FIGS. 1A and 1B schematically show a magnetic thin-film inductor developed for a 5 MHz switching DC-DC converter. As illustrated in FIG. 1A, the inductor has a sandwiched structure in which a coil (rectangular double spiral coil) 101 having a spiral pattern is sandwiched between interlayer insulation films 102 and the films 102 are sandwiched between soft magnetic thin films 103. As shown in FIG. 1B, the value Q (quality coefficient or performance coefficient or performance index) of the inductor is not so large but about 10 at most, due to eddy-current-losses of the coil 101, which is caused by high-frequency magnetic fluxes leaking from the soft magnetic thin films 103, is considered to be great. If the inductor is applied to a power supply, it is desirable that the value Q should be as large as possible. Under the present conditions, however, there are no reliable methods for increasing in value Q.
FIGS. 2A and 2B schematically show a trial product of a magnetic thin-film transformer. Referring to these figures, primary and secondary coils 201 and 202 are alternately wound on a magnetic thin film 204 with an insulation film 203 interposed therebetween. Both ends of the secondary coil 202 are connected to each other through Schottky barrier diodes 206a and 206b formed on a silicon substrate 205. Since the transformer requires two or more coils, its structure is complicated and few trial products have been made. Though a combination of magnetic fluxes of the coils is important, it has been hardly examined systematically. The transformer is not only useful for energy conversion but also widely applied to an electronic circuit since it has an impedance matching function; however, a practical transformer using a magnetic thin film is under development.
FIGS. 3A and 3B schematically show a prior art magnetic thin-film inductor which is being developed to serve as an input/output matching circuit of a power amplifier for mobile communication and a power supply choke. In the prior art inductor, an air-core spiral coil has been used as a lumped constant element in an MMIC (Monolithic Microwave Integrated Circuit); however, its area is large and thus its manufacturing cost is high. Recently, there have been many attempts to reduce the area of the element using a magnetic thin film in an inductor for a several-GHz band. As illustrated in FIG. 3A, the magnetic thin-film inductor is so constituted that a Ti/Au film 301 is sandwiched between magnetic thin films 302 of cobalt type granular films and the sandwiched structure is provided on an SiON film 303 formed on a silicon substrate 304. However, neither the analysis nor the design of the element has been established. It is thus difficult to examine a relationship between the characteristics and structure of the element in consideration of parasitic element components, an increase in high-frequency loss and the like. Under the present conditions, the above magnetic thin-film inductor is being developed by trial and error and has a long way to go before it is put to practical use (see FIG. 3B).
There is a case where a 1/4 wavelength transmission line transformer is used in an input/output matching circuit of a power amplifier for mobile communication. In this case, however, a dielectric substrate (whose dielectric constant is a value up to 8 and wavelength shortening rate is a value up to 3) such as alumina is often used in a propagation path of electromagnetic wave. For this reason, the line length of a 1/4 wavelength transmission line transformer of about 1 GHz is about 25 mm. If this transformer is mounted on an MMIC, it is difficult to make the MMIC monolithic since the area of the transformer is large.
A highly sensitized magnetic sensor utilizing a change in magnetic permeability due to an external magnetic field has recently been proposed (e.g., in M. Senda and Y. Koshimoto, 1997 INTERMAG Conference, GP-18). FIGS. 4A to 4C illustrate the above highly sensitized magnetic sensor. This magnetic sensor is so constituted that a magnetic core 404 including soft magnetic thin films (NiFe) 402 and dielectric films (SiO.sub.2) 403 which are alternately formed one on another, is provided so as to intersect with part of a conductor line 401 to sense a variation in impedance, which is proportional to a change in magnetic permeability of the soft magnetic thin films 402 due to an external magnetic field, as that of voltages at both ends of the conductor line 401. The impedance of a sensing section of the magnetic sensor is low. If, therefore, a 1/4 wavelength line 405 terminating at high resistance is coupled to the magnetic sensor, impedance matching can be achieved. Moreover, the variation in voltages due to that in external magnetic field can be made great by the boost effect of the line 405; however, in the magnetic sensor, the sensing section can be decreased in size but a peripheral circuit including the line 405 is complicated. Consequently, the peripheral circuit is larger than the sensing section and the entire sensor system is difficult to miniaturize.
As described above, a magnetic thin-film device, which is under development in order to make magnetic elements small and thin, has a lot of problems of characteristics, size and costs when it is used in a variety of fields.